1. Field of the Invention
The present invention relates to a photoelectric conversion device and an image pickup system using the photoelectric conversion device, and more particularly to a photoelectric conversion device including a plurality of pixels each having a photoelectric conversion area, a plurality of signal output lines through which electrical signals are to be read out from a plurality of pixels, and a plurality of amplification circuits which are provided in correspondence to a plurality of signal output lines in order to amplify the electrical signals and which includes at least one constant current circuit portion, and an image pickup system using the photoelectric conversion device.
2. Related Background Art
A photoelectric conversion device for amplifying and reading out signals from a plurality of pixels each having a photoelectric conversion area by column amplifiers, respectively, to read out successively the amplified signals from the column amplifiers by a scanning circuit is described in Japanese Patent Application Laid-Open No. H02-296470 for example.
FIG. 6 is a circuit diagram showing a configuration of a photoelectric conversion device described in Japanese Patent Application Laid-Open No. H02-296470. As shown in the drawing, a plurality of pixels for one column are connected to a column amplifier, as an amplification circuit, including a first MOS transistor M1 and a second MOS transistor M2. Thus, a signal from the pixel is inputted and amplified to be accumulated in the form of an offset signal (noise signal) and a sensor signal in capacitors CT1 and CT2 as temporary accumulation means, respectively.
Now, a large output resistance is required for a transistor constituting a constant current circuit in an inside of the column amplifier.
This reason is that if an operating current of the column amplifier changes, then a voltage drop in a common GND wiring into which the operating current is caused to flow changes accordingly (in FIG. 6, a voltage drop in Vvs changes) to generate crosstalk in signals in other columns. For example, when amplifiers for 1,000 columns are disposed in a horizontal direction, and an operating current of each amplifier is 10 μA, then a current of 10 mA in total is caused to flow into the GND wiring. In this case, if the operating currents fluctuate in the amplifiers for 900 columns to be decreased by 1 μA, the current of 0.9 mA in total changes accordingly. A change in operating current is developed in the form of fluctuation of the voltage drop due to a parasitic resistance to exert an influence on an operation for reading the signal.